This invention relates to high intensity arc discharge lamps and more particularly to high intensity arc discharge metal halide lamps having high efficacy.
Due to the ever-increasing need for energy conserving lighting systems that are used for interior and exterior lighting, lamps with increasing lamp efficacy are being developed for general lighting applications. Thus, for instance, arc discharge metal halide lamps are being more and more widely used for interior and exterior lighting. Such lamps are well known and include a light transmissive arc discharge chamber sealed about an enclosed a pair of spaced apart electrodes, and typically further contain suitable active materials such as an inert starting gas and one or more ionizable metals or metal halides in specified molar ratios, or both. They can be relatively low power lamps operated in standard alternating current light sockets at the usual 120 Volts rms potential with a ballast circuit, either magnetic or electronic, to provide a starting voltage and current limiting during subsequent operation.
These lamps typically have a ceramic material arc discharge chamber bounding a discharge region that usually contains quantities of metal halides such as CeI3 and NaI, (or PrI3 and NaI) and TlI, as well as mercury to provide an adequate voltage drop or loading between the electrodes, and also an inert ionization starting gas. A pair of electrodes is arranged on opposite ends of the discharge tube extending from outside the tube into the discharge region to allow electrical energization to occur in that region. Such lamps can have an efficacy as high as 145LPW at 250W with a Color Rendering Index (CRI) higher than 60, and with a Correlated Color Temperature (CCT) between 3000K and 6000K at 250W.
Referring to FIG. 1 in describing such a lamp in more detail, a typical arc discharge metal halide lamp, 10, known in the prior art is shown in a side view having a bulbous, transparent borosilicate glass envelope, 11, fitted into a conventional Edison-type metal base, 12. Lead-in, or electrical access, electrode wires, 14 and 15, of nickel or soft steel, each extend from a corresponding one of the two electrically isolated electrode metal portions in base 12 parallely through and past a borosilicate glass flare, 16, positioned at the location of base 12 and extending into the interior of envelope 11 along the axis of the major length extent of that envelope. Electrical access wires 14 and 15 extend initially on either side of, and in a direction parallel to, the envelope length axis past flare 16 to have portions thereof located further into the interior of envelope 11 with access wire 15 extending after some bending into a borosilicate glass dimple, 16′, at the opposite end of envelope 11. Electrical access wire 14 is provided with a second section in the interior of envelope 11, extending at an angle to the first section that parallels the envelope length axis, by having this second section welded at such an angle to the first section so that it ends after more or less crossing the envelope length axis.
Some remaining portion of access wire 15 in the interior of envelope 11 is bent at an obtuse angle away from the initial direction thereof parallel to the envelope length axis. Access wire 15 with this first bend therein past flare 16 directing it away from the envelope length axis, is bent again to have the next portion thereof extend substantially parallel that axis, and further along bent again at a right angle to have the succeeding portion thereof extend substantially perpendicular to, and more or less cross that axis near the other end of envelope 11 opposite that end thereof fitted into base 12. The succeeding portion of wire 15 parallel to the envelope length axis supports a conventional getter, 19, to capture gaseous impurities. Three additional right angle bends are provided further along in wire 15 to thereby place a short remaining end portion of that wire below and parallel to the portion thereof originally described as crossing the envelope length axis which short end portion is finally anchored at this far end of envelope 11 from base 12 in glass dimple 16′.
A ceramic arc discharge chamber, 20, configured about a bounded or contained region as a shell structure having polycrystalline alumina walls that are translucent to visible light, is shown in one of various possible geometric configurations in FIG. 1. Alternatively, the walls of arc discharge chamber 20 could be formed of aluminum nitride, yttria (Y2O3), sapphire (Al2O3), or some combinations thereof. Discharge chamber 20 is provided in the interior of envelope 11 which interior can otherwise either be evacuated, to thereby reduce the heat transmitted to the envelope from the chamber, or can instead be provided with an inert gaseous atmosphere such as nitrogen at a pressure greater than 300 Torr to thereby increase that heat transmission if operating the chamber at a lower temperature is desired. The region enclosed in arc discharge chamber 20 contains various ionizable materials, including metal halides and mercury which emit light during lamp operation and a starting gas such as the noble gases argon (Ar), xenon (Xe) or neon (Ne).
In this structure for arc discharge chamber 20, as better seen in the cross section view thereof in FIG. 2, a pair of polycrystalline alumina, relatively small inner and outer diameter truncated cylindrical shell portions, or capillary tubes, 21a and 21b, are each concentrically joined to a corresponding one of a pair of polycrystalline alumina end closing disks, 22a and 22b, about a centered hole therethrough so that an open passageway extends through each capillary tube and through the hole in the disk to which it is joined. These end closing disks are each joined to a corresponding end of a polycrystalline alumina tube, 25, formed as a relatively large diameter truncated cylindrical shell with that diameter designated as D, so as together to be about the enclosed region in providing the primary arc discharge chamber. The total length of the enclosed space in chamber 20 extends between the junctures of tubes 21a and 21b with the corresponding one of closing end disks 22a and 22b. The length of primary central portion chamber structure 25 of chamber 20 extends between the junctures therewith and each of closing end disks 22a and 22b. These various portions of arc discharge tube 20 are formed by compacting alumina powder into the desired shape followed by sintering the resulting compact to thereby provide the preformed portions, and the various preformed portions are joined together by sintering to result in a preformed single body of the desired dimensions having walls impervious to the flow of gases.
Chamber electrode interconnection wires, 26a and 26b, of niobium each extend out of a corresponding one of tubes 21a and 21b to reach and be attached by welding to, respectively, access wire 14 at its end portion crossing the envelope length axis and to access wire 15 at its portion first described as crossing the envelope length axis. This arrangement results in chamber 20 being positioned and supported between these portions of access wires 14 and 15 so that its long dimension axis approximately coincides with the envelope length axis, and further allows electrical power to be provided through access wires 14 and 15 to chamber 20.
FIG. 2 shows the discharge region contained within the bounding walls of arc discharge chamber 20 that are provided by structure 25, disks 22a and 22b, and tubes 21a and 21b of FIGS. 1 and 2, and FIG. 3 shows in cross section view the electrode arrangement having capillary tube 21a and the corresponding electrode extending therethrough into the discharge region in greater detail. Chamber electrode interconnection wire 26a, being of niobium, has a thermal expansion characteristic that relatively closely matches that of tube 21a and that of a glass frit, 27a, affixing wire 26a to the inner surface of tube 21a (and hermetically sealing that interconnection wire opening with wire 26a passing therethrough) but cannot withstand the resulting chemical attack resulting from the forming of a plasma in the main volume of chamber 20 during operation. Thus, a molybdenum lead-through wire, 29a, which can withstand operation in the plasma, is connected to one end of interconnection wire 26a by welding where this end is also surrounded by a portion of frit 27a in a hermetic seal, and the other end of lead-through-wire 29a is connected to one end of a tungsten main electrode shaft, 31a, by welding.
In addition, a tungsten electrode coil, 32a, is integrated and mounted to the tip portion of the other end of first main electrode shaft 31a by welding, so that an electrode, 33a, is configured by main electrode shaft 31a and electrode coil 32a. Electrode 33a is formed of tungsten for good thermionic emission of electrons while withstanding relatively well the chemical attack of the metal halide plasma. Lead-through wire 29a serves to dispose electrode 33a at a predetermined position in the region contained in the main volume of arc discharge chamber 20. This configuration results in lower temperatures in the sealing regions in capillary tube 21a during lamp operation electrode since 33a, in extending through this capillary tube into the chamber discharge region a significant distance, thereby spaces it, and the discharge arc established between this and the opposite end electrode during operation, further from the seal region in capillary tube 21a. 
Lead-through wire 29a and a portion of first main electrode shaft 31a are spaced from tube 21a by a molybdenum coil, 34a, having one end thereof in frit 27a. Since tungsten rod 31a with electrode coil 32a mounted thereon to form electrode 33a must be placed in the corresponding end of capillary tube 21a and then positioned to extend into the discharge region in arc discharge chamber 20a selected distance after the fabrication of that chamber has been completed, the inner diameter of capillary tube 21a and closing end disk 22a must have inner diameters exceeding the outer diameter of the electrode coil 32a. As a result, there is a substantial annular space between the outer surface of tungsten rod 31a and the inner surfaces of capillary tube 21a which must be taken up in part by the provision of molybdenum coil 34a around and against the corresponding portion of tungsten rod 31a, and which also extends to be around and against rod 26a, to complete the interconnections thereof and reduce the condensation in these regions of the metal halide salts occurring in chamber 20 during lamp operation. A typical diameter of interconnection wire 26a is 0.9 mm, and a typical diameter of electrode shaft 31a is 0.5 mm.
Similarly, in FIG. 2, chamber electrode interconnection wire 26b is affixed by a glass frit, 27b, to the inner surface of tube 21b (and hermetically sealing that interconnection wire opening with wire 26b passing therethrough). A molybdenum lead-through wire, 29b, is connected to one end of interconnection wire 26b by welding where this end is also surrounded by a portion of frit 27b in a hermetic seal, and the other end of lead-through wire 29b is connected to one end of a tungsten main electrode shaft, 31b, by welding. A tungsten electrode coil, 32b, is integrated and mounted to the tip portion of the other end of the first main electrode shaft 31b by welding, so that an electrode, 33b, is configured by main electrode shaft 31b and electrode coil 32b which is disposed at a predetermined position in the discharge region of chamber 20 to thereby provide sufficiently lower temperatures in the corresponding seal region. Lead-through wire 29b and a portion of second main electrode shaft 31b are spaced from tube 21b by a molybdenum coil, 34b, to fill in part the resulting annular space therebetween needed to allow electrode 33b to pass, the outer end of that coil also being in frit 27b. A typical diameter of interconnection wire 26b is also 0.9 mm, and a typical diameter of electrode shaft 31 is again 0.5 mm.
These electrode arrangements have “compromise” properties components in the seal regions within capillary tubes 21a and 21b, these being outer electrode portion niobium rods 26a and 26b which provide very good thermal expansion matching to the polycrystalline alumina but which are also subject to chemical attack during lamp operation by the metal halides within arc discharge tube 20. The exposure length of each of these outer electrode portions within arc discharge chamber 20 must be limited thus requiring the presence of the bridging middle part of the electrode arrangement, usually a molybdenum rod as above or a cermet rod, between such outer electrode portion and the corresponding tungsten electrode portion.
Care must also taken to ensure that the melted sealing frits 27a and 27b flow completely around and beyond the corresponding niobium rods to thereby form a protective surface over the niobium against the chemical reactions due to the halides. The frit flow length inside the corresponding capillary tube needs to be controlled very precisely. If the frit length is short, the niobium rod portion of the electrode is exposed to chemical attack by the halides. If this length is excessive, the large thermal mismatch between the frit and the solid middle electrode portion molybdenum, tungsten or cermet rod following inward from the niobium rod leads to cracks in the sealing frit or polycrystalline alumina, or both, in that location. Furthermore, although frits 27a and 27b are relatively resistant to halide attack during lamp operation, these sealing frits are not impervious to chemical attacks.
In these circumstances, of course, other ceramic arc discharge chamber constructions for ceramic metal halide lamps that make use of different sealing methods have been resorted to. These include methods such as direct sintering of polycrystalline alumina to the electrode arrangement, the use of cermets and grade temperature coefficient of expansion seals, or even the use of new arc tube materials that enable straight sealing of the tube body to a single material electrode such as molybdenum or tungsten. There have been occasional introduction of lamps that used a cermet to replace niobium.
However, these alternative methods have not yet been able to demonstrate an overall advantage with respect to improved lamp performance, lower cost, or compatibility with existing lamp factory processes. Thus, there is a desire to substitute some other material for niobium at the seal location so that arc discharge chamber electrode fabrication and the subsequent sealing process used therewith can be simplified and made more resistant to halide based chemical corrosion during lamp operation, and also allow a minimum and non-critical exposure length for the sealing frit used within the electrode capillary tubes.